1. Field of the Invention
The present invention relates to a passive mode-locked semiconductor laser diode, and an optical clock signal extracting device using the diode.
2. Description of Related Art
In an optical communication network, longer-distance and larger-capacity transmission has been developed. Along with development of the longer-distance transmission, optical losses on an optical transmission line, reduction in signal-to-noise ratio, wherein the reduction is caused by use of multistage optical amplifier, the group velocity dispersion of an optical fiber, and generation of a waveform strain depending on nonlinear optical-effects in the optical fiber have raised a problem that the quality of an optical signal is deteriorated. The larger transmission capacity has caused distortion generation of frequency and time-pulse waveforms to become a more remarkable problem.
Therefore, there have been provided repeaters at intervals of from several tens to a few hundred kilometers in the middle of the optical transmission line. In these repeaters, the frequency and time-pulse waveforms of an optical signal have been returned to the original shape, that is, so-called reproduction of the optical signal has been performed. One of main roles of these repeaters is extraction of a clock signal. Extraction of a clock signal is to generate a signal an optical pulse train in which a series of optical pulses are arranged on a time axis with a frequency corresponding to that of the bit rate, or a radio frequency (RF) signal, which is a sinusoidal electric signal, from an optical signal including optical pulses with strained time-pulse waveforms, that is, an optical signal which has so-called deteriorated quality.
The clock signal is, extracted as an electric signal in some cases, and as an optical signal in other cases. Hereinafter, as long as it is required to clearly specify which way is used for extraction, an electric clock signal and an optical clock signal are distinguished from each other in writing. Moreover, a frequency corresponding to a bit rate of an optical signal is assumed to indicate a frequency of f GHz when the bit rate of the optical signal is f Gbit/s. Hereinafter, the frequency corresponding to the bit rate of an optical signal is called a bit-rate frequency in some cases.
One of general techniques which have been known as a method by which clock signal is extracted is a method according to which an optical signal with deteriorated quality is configured to be input to a photodiode, and the like for photoelectric conversion, and an electric output signal from the photodiode is filtered by a bandpass filter for extraction of only frequency elements corresponding to the bit rate of the input optical signal. Hereinafter, an optical signal, including an optical signal with deteriorated quality, from which a clock signal is extracted is assumed to be called an input optical signal.
An optical pulse train, in which a series of optical pulses are arranged on a time axis with a period, as a repetition frequency, corresponding to the bit-rate frequency of an input optical signal, is generated by activating an optical pulsed laser diode such as a semiconductor laser, using an electric clock signal generated through a photodiode and a bandpass filter. Hereinafter, an optical signal is assumed to be a signal which is generated as a return-to-zero (RZ) signal by optical modulation of a train of optical pulses which are regularly arranged on a time axis at constant periodic intervals, wherein the RZ signal is a binary digital signal which is a send signal. On the other hand, the optical pulse train is assumed to indicate an integrated whole of optical pulses which are regularly arranged on a time axis at constant periodic intervals.
Generally, a clock signal may be stably extracted by using the photodiode even if there are time fluctuations on the plane of polarization of an input optical signal as the photoelectric conversion characteristics of the photodiode has reduced polarization dependence.
On the other hand, multiple transmission technologies such as optical time division multiplexing have been researched as a technology to increase the transmission capacity of an optical communication network. The bit rate of a multiple signal becomes very large because the bit rate of the multiple signal is equal to a result obtained by multiplying the bit rate per one channel among multiple ones by a factor of a number of channels.
When the bit rate of the multiple signal exceeds 40 Gbit/s, it becomes difficult for an electronic device to extract an clock signal. The reason is that a photodiode applicable even for an optical signal with a bit rate which is 40 Gbit/s or more, and an electric narrow-band filter applicable even for an electrical signal with a frequency of 40 Gbit/s or more have not been developed so far.
Conventionally, a first method through a fifth one, which will be explained later, have been examined in order to extract a clock signal from a high-speed, optical signal. That is, there have been examined methods by which an optical clock signal is extracted directly from an input optical signal without requiring a step at which photoelectric conversion of the input optical signal is performed, using a photodiode and the like.
The first method is a method (refer to, for example, Japanese Patent Publication No. 3510247 (corresponding to WO93/022855)) using a fiber-type passive mode-locked laser. In this method, an optical pulse signal is extracted by inputting an input optical signal according to repetition with near the bit-rate frequency of the input optical signal to the fiber-type passive mode-locked laser which generates optical pulses, and by synchronizing the generation frequency of the optical pulses by the fiber-type passive mode-locked laser and the bit-rate frequency of the input optical signal.
Though the second method (refer to, for example, T. Ono, T. Shimizu, Y. Yano, and H. Yokoyama, “Optical clock extraction from 10-Gbit/s data pulses by using monolithic mode-locked laser diodes”, OFC′ 95 Technical Digest, ThL4) has a similar configuration to that of the above-described first method, a passive mode-locked semiconductor laser diode, instead of the fiber-type passive mode-locked laser, is used in the second method. In the second method, an optical pulse train, in which a series of optical pulses are arranged at time intervals equal to the bit-rate frequency of an input optical signal, is generated by synchronizing orbiting optical pulses in the passive mode-locked laser and an optical pulse forming the input optical signal, based on modulation of an optical absorption coefficient in a saturable absorption region. The optical pulse train is an optical clock signal which has been extracted
The third method (refer to, for example, Japanese Patent Laid-Open Publication No. H11-326974) has a similar configuration to that of the above-described second method, and a passive mode-locked semiconductor laser diode is also used in the third method. However, the feature of the third method is that the passive mode-locked semiconductor laser diode is configured to include a coupling optical system such as a semiconductor optical amplifier, tunable filter, and lenses, and is a so-called external-cavity laser. Accordingly, the length of the resonator and the center wavelength of the tunable filter may be easily changed to bring about advantages that a frequency range in which optical clock signals may be extracted is wide, and the wavelengths of the extracted optical clock signals may be easily changed.
The fourth method (refer to, for example, Japanese Patent Laid-Open Publication No. 2001-94199) has a similar configuration to those of the above-described second and third methods, and a passive mode-locked semiconductor laser diode is also used in the fourth method. However, the feature of the fourth method is that there are used two passive mode-locked semiconductor laser diodes, that is, a first passive mode-locked semiconductor laser diode, and a second one. The first passive mode-locked semiconductor laser diode operates at a frequency near the bit-rate frequency of an input optical signal, and the second one operates at a frequency near a frequency which is an integral submultiple of the bit-rate frequency of an input optical signal. As the two passive mode-locked semiconductor laser diodes with different operating frequencies from each other are configured to be used as described above, it is possible to generate a frequency-divided optical clock signal with a frequency which is an integral submultiple of the bit-rate frequency of an input optical signal. Moreover, as a return-loop optical path is formed in the second passive mode-locked semiconductor laser diode, using an optical gate element, it is realized in the fourth method to generate a frequency-divided clock signal in a stable manner
The fifth method (refer to, for example, M. Jinno and T. Matsumoto, “All-optical timing extraction using a 1.5 μm self pulsating multielectrode DFB LD”, Electron, Lett., vol. 24, No. 23, pp. 1426-1427, 1988) is a method in which an optical pulse signal is extracted, using a self pulsating multielectrode distributed feed-back laser diode. An optical pulse signal is extracted by synchronizing an optical-pulse frequency generated from the self pulsating multielectrode distributed feed-back laser diode and the bit-rate frequency of the input optical signal after the bit-rate frequency of an input optical signal and the input optical signal are input to the self pulsating multielectrode distributed feed-back laser diode.
However, there is caused a problem, in the above-described first through fifth methods, that extracting operation of an optical clock signal strongly depends on the plane of polarization of the input optical signal. As an optical fiber before reaching an optical repeater is not processed in such a way that the plane of polarization of an input optical signal, which is propagating, is kept constant, the plane of polarization of the input optical signal generally causes time fluctuation. That is, the extracting operation of the optical clock signal becomes unstable by the time fluctuation of the plane of polarization of the input optical signal.
In the above-described first through fifth methods, there will be illustrated as follows a reason that the extracting operation of the optical clock signal strongly depends on the plane of polarization of the input optical signal.
An electro-optical Kerr effect is used in the first method, and the Kerr effect has a strong dependence on the plane of polarization. That is, though a direction on the plane of polarization of an optical pulse forming an optical clock signal existing in the inside of a fiber-type mode-locked laser and the direction on the plane of polarization of an input optical signal are in parallel with each other in some cases, and are orthogonal to each other in other cases, a coefficient of an electro-optical Kerr effect obtained in the parallel case is three times a coefficient in the orthogonal case. Accordingly, when the direction of the plane of polarization of the input optical signal is in accordance with that of the plane of polarization of oscillation light by a fiber-type mode-locked laser, an optical clock signal is extracted with a high efficiency. However, when the both directions of the planes of polarization are orthogonal to each other, there is caused a situation in which no optical clock signal is extracted.
The gain region of the passive mode-locked semiconductor laser diode, which is used in the second through the fifth methods, is realized by a bulk crystal layer, a quantum well layer, or a strained quantum well layer. Moreover, the saturable absorption region is realized by the quantum well layer or the strained quantum well layer. The reason will be described as follows.
That is, the reason is that it is easy to stabilize the mode-locking operation because absorption saturation energy may be reduced by a configuration in which the saturable absorption region includes the quantum well layer or the strained quantum well layer. Moreover, the reason is that a phenomenon in which high-speed saturable absorption occurs may be realized, which is preferable for extraction of an optical clock signal with a high bit-rate frequency.
The optical properties of the quantum well layer has a strong dependence on polarization. Especially, the strained quantum well layer into which no strain or compressive strain is introduced has characteristics such as a high optical gain, a large differential gain, a low α parameter, or low absorption saturation energy for polarization (so-called transverse electric wave (TE) polarization) parallel to the laminated surface of the quantum well. Accordingly, when an optical clock signal is extracted, using the passive mode-locked semiconductor laser diode in which the quantum well layer is adopted into the gain region and the saturable absorption region, the following preferable effects may be expected.
That is, when the plane of polarization of an input optical signal is TE polarization, the input optical signal is amplified with a high efficiency by optical amplification in the gain region. Moreover, as the absorption saturation energy is low in the saturable absorption region, there may be realized the larger modulation factor of an optical absorption coefficient in a saturable absorption region generated by the absorption saturation induced by the input optical signal. Thereby, even when the intensity of the input optical signal is low, a required modulation factor of the optical absorption coefficient in the saturable absorption region for extraction of the optical clock signal may be easily realized. That is, even when the intensity of the input optical signal is low, the optical clock signal may be extracted in a stable manner. Here, the modulation factor of the optical absorption coefficient is a change in the optical absorption coefficient, that is, a ratio between the minimum value and the maximum one of the optical absorption coefficient. The large modulation factor means that the degree of change in the optical absorption coefficient is large.
One the other hand, when the plane of polarization of the input optical signal is TE polarization, and when the plane is orthogonal polarization (so-called transverse magnetic wave TM polarization), the effect of optical amplification is not obtained in the gain region, and, moreover, the absorption saturation energy of the saturable absorption region is high. Accordingly, a required degree of modulation of the optical absorption coefficient in the saturable absorption region for extraction of the optical clock signal may not be realized. That is, when the optical clock signal is extracted from the input optical signal, there is a problem that there is generated the dependence on the plane of polarization of the input optical signal, that is, the optical clock signal is extracted in a stable manner when the input optical signal is TE polarization, and the optical clock signal may not be extracted in the case of the TM polarization.
There is no guarantee that the input optical signal is kept in the TE polarization because the optical fiber before reaching an optical repeater is not processed as described above in such a way that the plane of polarization of the input optical signal, which is propagating, is kept constant, and extraction of the optical clock signal depends on the plane of polarization of the input optical signal. That is, it is meant that the optical clock signal may not be extracted in a stable manner.
Then, a sixth and a seventh methods (refer to, for example, Japanese Patent Laid-Open Publication No. 2004-363873 (corresponding to U.S. Pat. No. 6,954,559) and Y. Hashimoto, R. Kuribayashi, S. Nakamura, and I. Ogura, “Optical clock recovery using optical phase-locked loop with voltage-controlled mode-locked semiconductor laser,” ECOC 2004, Vol. 3, paper We2.5.1.) have been examined in order to solve the problem related with the dependence on the plane of polarization of the input optical signal.
In the sixth method, the input optical signal is separated in the first place into a polarization element (called a TE polarization element) in accordance with the plane of polarization of oscillation light by the mode-locked semiconductor laser diode and an element (called a TM polarization element) having the plane of polarization orthogonal to the plane of polarization by the oscillation light. The TE polarization element is input to one end surface of the resonator in the mode-locked semiconductor laser, keeping the polarization state. The TM polarization element is input to the other end surface of the resonator in the mode-locked semiconductor laser after the polarization state is rotated by 90 degrees and the element is made in accordance with the plane of polarization of oscillation light by the mode-locked semiconductor laser diode. The optical clock signal is extracted by the above-described method without depending on the plane of polarization of the input optical signal.
In the seventh method, the optical clock signal is extracted, using a semiconductor optical amplifier (SOA) as a phase comparator. As SOA functioning without depending on the plane of polarization is used, the optical clock signal is extracted without depending on the plane of polarization of the input optical signal.
However, a device, which is provided with a number of components such as a polarization synthetic separation circuit, an optical delay device, a specially specified SOA, a photodiode, and the like, and has a complex structure, is required for the sixth or the seventh method. Industrially, it is the most preferable to extract the optical clock signal in a stable manner without depending on the plane of polarization of the input optical signal, using a simple device which may be realized by a single element. If the optical clock signal is realized by the single element, maintenance of the simple device is more easily performed, and stable operation is more reliably executed in comparison with a case in which the above-described device, which is provided with a number of components, and has a complex structure, is used.
If there is developed a single element which has a function by which an optical clock signal may be extracted from an input optical signal in a stable manner without depending on the plane of polarization of the input optical signal, the above element may be inexpensively provided, using a technology by which a high degree of integration and mass production of a semiconductor are realized.
Accordingly, an object of the present invention is to provide a passive mode-locked semiconductor laser diode and an optical clock signal extracting device including the above laser diode, wherein the passive mode-locked semiconductor laser diode is an element with a simple configuration having properties similar to those of a single element, and may extract, from an input optical signal without depending on the polarization direction of the input optical signal, an optical clock signal with a repetition frequency corresponding to the bit-rate frequency (called “clock frequency” in some times) of the input optical signal. Moreover, another object of the present invention is to provide a method by which the above passive mode-locked semiconductor laser diode and the optical clock signal extracting device are activated.